Flexible, high purity expanded graphite sheet, method of producing same, and carbon crucible lining using said sheet

ABSTRACT

The invention provides a flexible, highly pure expanded graphite sheet characterized by having an impurity content of 10 ppm or less and such a degree of flexibility that a sample thereof, 10×100 mm in size can withstand at least 10 times of bending in flexibility test comprising repeatedly bending the sample, with a 50-g weight suspended from one end thereof, by means of bending bodies with a diameter of 6 mm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flexible, high-purity expandedgraphite sheet, and to a method of producing the same.

2. Description of the Prior Art

Expanded graphite sheets are generally produced by treating naturalgraphite, pyrolytic graphite, kish graphite or the like with a mixedsolution containing sulfuric acid and nitric acid, for instance, thenwashing the same with water, drying the same, treating the same forexpansion in an expansion oven at about 1,000° C., and firming the sameinto sheets using a rolling machine, for instance. Expanded graphitesheets are excellent in heat resistance and in gas and liquid inimpermeability and therefore are used as packing materials, valvesheets, gaskets, and fuel cell separators, among others.

Japanese Patent No. 2,620,606 discloses that highly pure expandedgraphite sheets having an impurity content of not more than 15 ppm canbe obtained by treating such expanded graphite sheets for increasedpurity in a halogen gas atmosphere at 2,000° C. or above. Such sheetsare used in the process of semiconductor production.

Hereinafter, a detailed description is given of a high-purity expandedgraphite sheet for use in semiconductor production, which is taken as anexample. Such a high-purity expanded graphite sheet is also used in theCzochralski (hereinafter referred to as “CZ” for short) process, whichis a representative single crystal pulling technique. A sectional viewof the main parts of a CZ apparatus is shown in FIG. 1. The CZ apparatuscomprises such parts as a carbon crucible 5 supporting a quarts crucible1, a heater 2, an upper ring 6, and an inner shield 7, among others. Inthe CZ apparatus, polycrystalline silicon placed in the quartz crucible1 is heated to a high temperature to give a silicon melt 3, and the tipof a seed crystal held by a seed chuck is brought into contact with theraw material melt 3 and then pulled up while maintaining the contact tothereby pull up a silicon single crystal 4.

As shown in FIG. 1, the carbon crucible 5 made of graphite or a carbonfiber-reinforced carbon composite material (such crucible is hereafterreferred to as “carbon crucible”) is in direct contact with the quartzcrucible 1 and, therefore, the surface of the carbon crucible 5 isgradually converted to silicon carbide (hereinafter referred to as“SiC”) as a result of the reaction between the quartz crucible 1 andcarbon crucible 5 and/or the reaction between vaporized silicon and thegraphite crucible. The difference in coefficient of thermal expansionbetween carbon and SiC is conducive to cracking of the carbon crucible,for instance. Furthermore, the quartz crucible 1 becomes firmly stickingto the carbon crucible 5, making it difficult to take out the quartzcrucible 1.

Japanese Patent No. 2,528,285 discloses, as a means for solving suchproblems, the use of a high-purity expanded graphite sheet as a linerintervening between the quartz crucible 1 and carbon crucible 5.

When an expanded graphite sheet is treated for improving the puritythereof, the flexibility of the expanded graphite sheet is generallyimpaired, so that it can no loner be used as a member which is requiredto have flexibility. Therefore, Japanese Patent No. 2,620,606 disclosesa method of restoring flexibility which comprises compression molding.However, the method has other problems; the purity of the high-purityexpanded graphite sheet is decreased upon compression molding and, whena complicated shape is given to the high-purity expanded graphite sheetinsufficient in flexibility restoration by working with a cutter, forinstance, the peripheral parts of the sheet are subject to crackingand/or chipping.

When the whole inside surface (if the carbon crucible is covered with anexpanded graphite sheet, the efficiency of heating of the quartzcrucible 1 decreases. Therefore, in recent years, various complicatedliner shapes have been proposed so that the quartz crucible heatingefficiency may be improved. Basically, each single crystal productionoperation consumes one intervening liner and, therefore, it is importantto provide a method of producing high-purity expanded graphite sheetswhich is excellent in mass productivity.

Accordingly, it is an object of the present invention to provide ahigh-purity expanded graphite sheet having flexibility and a method ofproducing the same. Another object of the invention is to provide amethod of manufacturing expanded graphite sheets which is suited formass production as well.

SUMMARY OF THE INVENTION

The present inventors made intensive investigations in an attempt toaccomplish the above objects and, as a result, found that when anexpanded graphite sheet whose bulk density is within a certain specificrange is treated for attaining high purity, a high-purity expandedgraphite sheet can be obtained without deterioration in flexibility evenafter the treatment for attaining high purity. This and other findingshave now led to completion of the present invention. Thus, in a firstaspect (Claim 1), the invention provides a flexible, high-purityexpanded graphite sheet characterized in that it has an impurity contentnot exceeding 10 ppm and has such a degree of flexibility that a samplethereof can withstand at least 10 times of bending on a testingapparatus such as shown in FIG. 4. In a second aspect (Claim 2), theinvention provides a flexible, high-purity expanded graphite sheet whichhas the above characteristics and is further characterized by its bulkdensity being 0.5 to 1.3 g/cm³. In a third aspect (Claim 3), theinvention provides a flexible, high-purity expanded graphite sheet asdefined in Claim 1 or 2 which is further characterized by its thicknessbeing 0.2 to 1.0 mm.

For obtaining the flexible, high-purity expanded graphite sheetaccording to Claim 1, it is necessary to use scaly graphite, kishgraphite, pyrolytic graphite or the like as a filler, subject this tooxidation treatment by immersing the same in a mixed acid supplementedwith concentrated sulfuric acid, concentrated nitric acid, etc., washthe immersed filler with water and, after drying, treat the same forexpansion by heating to give expanded graphite, and adjust the bulkdensity thereof to about 0.7 to 1.3 g/cm³ by compression molding using apress or rolling machine. Expanded graphite sheets having a bulk densitylower than 0.7 g/cm³ cannot acquire, even after purification treatment,such a degree of flexibility as to withstand at least 10 times ofbending, hence are unfavorable. Expanded graphite sheets having a bulkdensity exceeding, 1.3 g/cm³ are also undesirable, since the impuritycontent therein cannot be reduced to 10 ppm or below even bypurification treatment. In a preferred embodiment of the invention,expanded graphite sheets with a bulk density adjusted to 0.8 to 1.3g/cm³, more preferably to 0.9 to 1.3 g/cm³, are subjected to treatmentfor attaining high purity. The method of purification treatment itselfmay be any of those known in the art. For example, flexible, high-purityexpanded graphite sheets having an impurity content of not more than 10ppm and a degree of flexibility as to withstand at least 10 times ofbending can be obtained by heating expanded graphite sheets at 2,000° C.or above in a halogen gas atmosphere to thereby convert metals in thesheets to metal halide compounds showing a high vapor pressure and allowthem to vaporize. The bulk density and sheet thickness of expandedgraphite sheets after purification treatment show little changes andremain almost equal to those before treatment. A flexibility measuringapparatus is schematically shown in FIG. 3. A 50-g weight 22 is attachedto one end of a 10 mm×100 mm sample 21 of a high-purity expandedgraphite sheet and subjected to repeated bending by means of bendingbodies 24 with a diameter of 6 mm. The number of times of bending untilbreakage is counted and recorded as the flexibility in the longitudinaldirection.

In a fourth aspect (Claim 4), the invention provides a method ofproducing flexible, high-purity expanded graphite sheets which comprisesforming an expanded graphite sheet having a bulk density of 0.7 to 1.3g/cm³ into a desired shape and then subjecting the shaped sheet topurification treatment. Expanded graphite sheets having a bulk densityless than 0.7 g/cm³ are undesirable since any purification treatmentcannot improve their flexibility to a level of at least 10 times, asmentioned above. Expanded graphite sheets having a bulk densityexceeding 1.3 g/cm³ are also unfavorable since their impurity contentcannot be reduced to 10 ppm or less by purification treatment.Preferably, the expanded graphite sheet to be treated for purificationhas a thickness of 0.2 to 1.0 mm. When the expanded graphite sheetbefore purification treatment is less than 0.2 mm in thickness, thesheet after purification treatment tends to show marked decreases inflexibility and strength, hence are susceptible to cracking, forinstance. When the expanded graphite sheet is thicker than 1.0 mm, theimpurity content cannot be reduced to a satisfactory extent bypurification treatment. More preferably, the expanded graphite sheet tobe subjected to purification treatment has a thickness of 0.3 to 0.9 mm.Most preferably, 0.5 to 0.8-mm-thick expanded graphite sheets aresubjected to purification treatment. As the “desired shape” so referredto herein, there may be mentioned, for example, the shape shown in FIG.2 or FIG. 3 and, further, figures of line symmetry, such as ellipses,stars and the Imperial crest of chrysanthemum, and asymmetric figures.

In a fifth aspect (Claim 5), the invention provides a modification ofthe method specified above which modification comprises forming aplurality of expanded graphite sheets each into one and the same desiredshape simultaneously in a laminate form, followed by purificationtreatment. This modification is preferred since the mass productivity inmanufacturing expanded graphite sheets having one and the same desired,complicated shape can be improved by laying a plurality of expandedgraphite sheets one upon another and working them simultaneously in oneoperation according to the method mentioned above. Preferably, aplurality of expanded graphite sheets each having a thickness of 0.2 to1.0 mm are laid one upon another. When each expanded graphite sheet isless than 0.2 mm in thickness, the expanded graphite sheet cannotacquire flexibility or strength but becomes susceptible to cracking, forinstance. When each sheet is thicker than 1.0 mm, the impurity contentcannot be reduced to a satisfactory extent by purification treatment;this is unfavorable. It is desirable that the bulk density of expandedgraphite sheets prior to purification treatment be 0.7 to 1.3 g/cm³.

In a sixth aspect (Claim 6), the invention provides a method ofproducing such expanded graphite sheets as mentioned above in which theshaping of expanded graphite sheets is carried out by at least onemethod selected from among such methods as slitting, punching using aThomson die, water jet working, and laser working. From the massproductivity viewpoint, however, punching with a Thomson die and waterjet working are preferred since they can reduce the working time, areless likely to decrease the purity of expanded graphite sheets and,further, are excellent in mass productivity. It is to be added that whenexpanded graphite sheets are shaped by punching using a Thomson die, thepurification treatment should preferably be carried out after removingworking dust from the worked surface (cut surface) by ultrasoniccleaning in a state immersed in water, alcohol or the like and thesubsequent drying for evaporation of the moisture.

In a seventh aspect (Claim 7), the invention provides the use of theflexible, high-purity expanded graphite sheet as defined in any one ofClaims 1 to 3 as a carbon crucible liner. When such a high-purityexpanded graphite sheet having flexibility and a low impurity content isused as a carbon crucible liner, the stability of the quartz crucible isgood and the intrusion of silicon monoxide gas can be prevented, so thatthe carbon crucible can be inhibited from beg converted to siliconcarbide and the life of the expensive carbon crucible can be prolonged.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a single crystal puller.

FIG. 2 is an illustration of shaping of a high-purity expanded graphitesheet suited for use as an intervening liner in a single crystal pullingapparatus.

FIG. 3 is an illustration of shaping of another high-purity expandedgraphite sheet suited for use as a liner in a single crystal puller.

FIG. 4 is a schematic representation of a flexibility measuringapparatus.

In FIGS. 1 and 4, the numerical symbols respectively denote thefollowing: 1—quartz crucible, 2—heater, 3—silicon melt, 4—silicon singlecrystal, 5—carbon crucible, 6—upper ring, 7—inner shield, 8—lower ring,9—bottom heater, 10—heat insulator, 11—spill tray, 21—sample (ofhigh-purity expanded graphite sheet), 22—weight, 23—directions ofbending, 24—benders

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples illustrate the present invention morespecifically. These examples are, however, by no means limitative of thescope of the present invention.

Example 1

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-50, size: 1,000×1,000×0.5 (mm), bulk density: 1.3 g/cm³, ash content:0.2% by mass) was obtained. This sheet was formed into the shape shownin FIG. 2 by punching using a hydraulic press and a Thomson die at apressure of 10 MPa. The shaped article was then treated forpurification; thus, it was maintained at 2,000° C. for 10 hours whilegaseous chlorine was fed.

Example 2

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 1. This laminate was formed into the shapeshown in FIG. 2 by punching using a hydraulic press and a Thomson die ata pressure of 50 MPa. The shaped article was then treated forpurification; thus, it was maintained at 2,000° C. for 10 hours whilegaseous chlorine was fed.

Example 3

An expanded graphite sheet made by Toyo Tanso Co. Ltd. grade name:PF-40, thickness 0.4 mm, bulk density: 1.0 g/cm³, ash content: 0.2% bymass) was obtained. This sheet was formed into the shape shown in FIG. 2by punching using a hydraulic press and a Thomson die at a pressure of10 MPa. The shaped article was then treated for purification; thus, itwas maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 4

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 3. This laminate was formed into the shapeshown in FIG. 2 by punching using a hydraulic press and a Thomson die ata pressure of 50 MPa. The shaped article was then treated forpurification; thus, it was maintained at 2,000° C. for 10 hours whilegaseous chlorine was fed.

Example 5

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-40, thickness: 0.4 mm, bulk density: 0.7 g/cm³, ash content: 0.2% bymass) was obtained. This sheet was formed into the shape shown in FIG. 2by punching using a hydraulic press and a Thomson die at a pressure of10 MPa. The shaped article was then treated for purification; thus, itwas maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 6

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 5. This laminate was formed into the shapeshown in FIG. 2 by punching using a hydraulic press and a Thomson die ata pressure of 50 MPa. The shaped article was then treated forpurification; thus, it was maintained at 2,000° C. for 10 hours whilegaseous chlorine was fed.

Example 7

An expanded graphite sheet made of the same material and having the samesize as that used in Example 1 was obtained. This sheet was formed intothe shape shown in FIG. 2 by subjecting to water jet working using anozzle having an orifice diameter of 0.1 mm at a water pressure of 3,000kg/cm². The shaped article was then treated for purification; thus, itwas maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 8

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 1. This laminate was formed into the shapeshown in FIG. 2 by subjecting to water jet working using a nozzle havingan orifice diameter of 0.1 mm at a water pressure of 3,000 kg/cm². Theshaped article was then treated for purification; thus, it wasmaintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 9

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-40, thickness: 0.4 mm, bulk density: 1.0 g/cm³, ash content: 0.2% bymass) was obtained. This sheet was formed into the shape shown in FIG. 2by subjecting to water jet working using a nozzle having an orificediameter of 0.1 mm at a water pressure of 3,000 kg/cm². The shapedarticle was then treated for purification; thus, it was maintained at2,000° C. for 10 hours while gaseous chlorine was fed.

Example 10

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 5. This laminate was formed into the shapeshown in FIG. 2 by subjecting to water jet working using a nozzle havingan orifice diameter of 0.1 mm at a water pressure of 3,000 kg/cm². Theshaped article was then treated for purification; thus, it was heated toand maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 11

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-40, thickness: 0.4 mm, bulk density: 0.7 g/cm³, ash content: 0.2% bymass) was obtained. This sheet was formed into the shape shown in FIG. 2by subjecting to water jet working using a nozzle having an orificediameter of 0.1 mm at a water pressure of 3,000 kg/cm². The shapedarticle was then treated for purification; thus, it was maintained at2,000° C. for 10 hours while gaseous chlorine was fed.

Example 12

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 5. This laminate was formed into the shapeshown in FIG. 2 by subjecting to water jet working using a nozzle havingan orifice diameter of 0.1 mm at a water pressure of 3,000 kg/cm². Theshaped article was then treated for purification; thus, it wasmaintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 13

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-20, size: 1,000×1,000×0.2 (mm), bulk density: 1.3 g/cm³, ash content:0.2% by mass) was obtained. This sheet was formed into the shape shownin FIG. 2 by punching using a hydraulic press and a Thomson die at apressure of 10 MPa. The shaped article was then treated forpurification; thus, it was maintained at 2,000° C. for 10 hours whilegaseous chlorine was fed.

Example 14

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 13. This laminate was formed into the shapeshown in FIG. 2 by punching using a hydraulic press and a Thomson die ata pressure of 50 MPa. The shaped article was then treated forpurification; thus, it was maintained at 2,000° C. for 10 hours whilegaseous chlorine was fed.

Example 15

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-20, thickness: 0.2 mm, bulk density: 1.0 g/cm³, ash content 0.2% bymass) was obtained. This sheet was formed into the shape shown in FIG. 2by punching using a hydraulic press and a Thomson die at a pressure of10 MPa. The shaped article was then treated for purification; thus, itwas maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 16

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 15. This laminate was formed into the shapeshown in FIG. 2 by punching using a hydraulic press and a Thomson die ata pressure of 50 MPa. The shaped article was then treated forpurification; thus, it was maintained at 2,000° C. for 10 hours whilegaseous chlorine was fed.

Example 17

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-20, thickness: 0.2 mm, bulk density: 0.7 g/cm³, ash content: 0.2% bymass) was obtained. This sheet was formed into the shape shown in FIG. 2by punching using a hydraulic press and a Thomson die at a pressure of10 MPa. The shaped article was then treated for purification; thus, itwas maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 18

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 17. This laminate was formed into the shapeshown in FIG. 2 by punching using a hydraulic press and a Thomson die ata pressure of 50 MPa. The shaped article was then treated forpurification; thus, it was maintained at 2,000° C. for 10 hours whilegaseous chlorine was fed.

Example 19

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-100, size: 1,000×1,000×1.0 (mm), bulk density: 1.3 g/cm³, ashcontent: 0.2% by mass) was obtained. This sheet was formed into theshape shown in FIG. 2 by subjecting to water jet working using a nozzlehaving an orifice diameter of 0.1 mm at a water pressure of 3,000kg/cm². The shaped article was then treated for purification; thus, itwas maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 20

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 19. This laminate was formed into the shapeshown in FIG. 2 by subjecting to water jet working using a nozzle havingan orifice diameter of 0.1 mm at a water pressure of 3,000 kg/cm². Theshaped article was then treated for purification; thus, it was heated toand maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 21

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-100, thickness: 1.0 mm, bulk density: 1.0 g/cm³, ash content: 0.2% bymass) was obtained. This sheet was formed into the shape shown in FIG. 2by subjecting to water jet working using a nozzle having an orificediameter of 0.1 mm at a water pressure of 3,000 kg/cm². The shapedarticle was then treated for purification; thus, it was maintained at2,000° C. for 10 hours while gaseous chlorine was fed.

Example 22

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 21. This laminate was formed into the shapeshown in FIG. 2 by subjecting to water jet working using a nozzle havingan orifice diameter of 0.1 mm at a water pressure of 3,000 kg/cm². Theshaped article was then treated for purification; thus, it was heated toand maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 23

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-100, thickness: 1.0 mm, bulk density: 0.7 g/cm³, ash content: 0.2% bymass) was obtained. This sheet was formed into the shape shown in FIG. 2by subjecting to water jet working using a nozzle having an orificediameter of 0.1 mm at a water pressure of 3,000 kg/cm². The shapedarticle was then treated for purification; thus, it was maintained at2,000° C. for 10 hours while gaseous chlorine was fed.

Example 24

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 23. This laminate was formed into the shapeshown in FIG. 2 by subjecting to water jet working using a nozzle havingan orifice diameter of 0.1 mm at a water pressure of 3,000 kg/cm². Theshaped article was then treated for purification; thus, it was heated toand maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 25

A laminate was prepared by laying, one upon another, 5 expanded graphitesheets made of the same material and having the same size as the sheetused in Example 1. This laminate was formed into the shape shown in FIG.2 by subjecting to water jet working using a nozzle having an orifice

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-100, thickness: 1.0 mm, bulk density: 1.0 g/cm³, ash content: 0.2% bymass) was obtained. This sheet was formed into the shape shown in FIG. 2by subjecting to water jet working using a nozzle having an orificediameter of 0.1 mm at a water pressure of 3,000 kg/cm². The shapedarticled was then treated for purification; thus, it was maintained at2,000° C. for 10 hours while gaseous chlorine was fed.

Example 22

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 21. This laminate was formed into the shapeshown in FIG. 2 by subjecting to water jet working using a nozzle havingan ode diameter of 0.1 mm at a water pressure of 3,000 kg/cm². Theshaped article was then treated for purification; thus, it was heated toand maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 23

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-100, thickness: 1.0 mm, bulk density: 0.7 g/cm³, ash content: 0.2% bymass) was obtained. This sheet was formed into the shape shown in FIG. 2by subjecting to water jet working using a nozzle having an orificediameter of 0.1 mm at a water pressure of 3,000 kg/cm². The shapedarticle was then treated for purification; thus, it was maintained at2,000° C. for 10 hours while gaseous chlorine was fed.

Example 24

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Example 23. This laminate was formed into the shapeshown in FIG. 2 by subjecting to water jet working using a nozzle havingan orifice diameter of 0.1 mm at a water pressure of 3,000 kg/cm². Theshaped article was then treated for purification; thus, it was heated toand maintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

Example 25

A laminate was prepared by laying, one upon another, 5 expanded graphitesheets made of the same material and having the same size as the sheetused in Example 1. This laminate was formed into the shape shown in FIG.2 by subjecting to water jet working using a nozzle having an orificedie at a pressure of 50 MPa. The shaped article was then treated forpurification; thus, it was maintained at 2,000° C. for 10 hours whilegaseous chlorine was fed.

Comparative Example 5

An expanded graphite sheet made by Toyo Tanso Co. Ltd. (grade name:PF-120, size: 1,000×1,000×1.2 (mm), bulk density: 1.3 g/cm³, ashcontent: 0.2% by mass) was obtained. This sheet was formed into theshape shown in FIG. 2 by punching using a hydraulic press and a Thomsondie at a pressure of 10 MPa. The shaped article was then treated forpurification; thus, it was maintained at 2,000° C. for 10 hours whilegaseous chlorine was fed.

Comparative Example 6

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made of the same material and having the same size asthe sheet used in Comparative Example 5. This laminate was formed intothe shape shown in FIG. 2 by punching using a hydraulic press and aThomson die at a pressure of 50 MPa. The shaped article was then treatedfor purification; thus, it was maintained at 2,000° C. for 10 hourswhile gaseous chlorine was fed.

Comparative Example 7

A laminate was prepared by laying, one upon another, 5 expanded graphitesheets made of the same material and having the same size as the sheetused in Example 1. This laminate was formed into the shape shown in FIG.2 by manual working using a cutter. The time required for the shapingwas 1 hour. The shaped article was subjected to 3 minutes of ultrasoniccleaning at 43 kHz to thereby remove cut dust derived from the expandedgraphite laminate. Then, the shaped article was dried at 100° C. for 30minutes to evaporate the moisture and then treated for purification;thus, it was maintained at 2,000° C. for 10 hours while gaseous chlorinewas fed.

Comparative Example 8

A laminate was prepared by laying, one upon another 10 expanded graphitesheets made by Toyo Tanso Co. Ltd. (grade name: PF-40, size:1,000×1,000×0.4 (mm), bulk density: 0.6 g/cm³, ash content: 0.2% bymass). This laminate was formed into the shape shown in FIG. 2 bysubjecting to water jet working using a nozzle having an orificediameter of 0.1 mm at a water pressure of 3,000 kg/cm². The shapedarticle was then treated for purification; thus, it was maintained at;2,000° C. for 10 hours while gaseous chlorine was fed.

Comparative Example 9

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made by Toyo Tanso Co. Ltd. (grade name: PF-40, size:1,000×1,000×0.4 (mm), bulk density: 0.3 g/cm³, ash content: 0.2% bymass). This laminate was formed into the shape shown in FIG. 2 bysubjecting to water jet working using a nozzle having an orificediameter of 0.1 mm at a water pressure of 3,000 kg/cm². The shapedarticle was then treated for purification; thus, it was maintained at2,000° C. for 10 hours while gaseous chlorine was fed.

Comparative Example 10

A laminate was prepared by laying, one upon another, 10 expandedgraphite sheets made by Toyo Tanso Co. Ltd. (grade name: PF-40, size:1,000×1,000×0.4 (mm), bulk density: 0.2 g/cm³, ash content: 0.2% bymass). This laminate was formed into the shape shown in FIG. 2 bysubjecting to water jet working using a nozzle having an orificediameter of 0.1 mm at a water pressure of 3,000 kg/cm². The shapedarticle was then treated for purification; thus, it was maintained at2,000° C. for 10 hours while gaseous chlorine was fed.

Comparative Example 11

An expanded graphite sheet made by Toyo Tanso Co. Ltd. grade name:PF-40, size: 1,000×1,000×0.4 (mm), bulk density: 0.6 g/cm³, ash content:0.2% by mass) was obtained. This sheet was formed into the shape shownin FIG. 2 by subjecting to water jet working using a nozzle having anorifice diameter of 0.1 mm at a water pressure of 3,000 kg/cm². Theshaped article was then treated for purification; thus, it wasmaintained at 2,000° C. for 10 hours while gaseous chlorine was fed.

For each of Examples 1 to 25 and Comparative Examples 1 to 11, the bulkdensity of the expanded graphite sheet, the thickness per sheet, thenumber of sheets in the laminate, the method of shaping, the flexibilityin the longitudinal direction, and the purity after purificationtreatment are summarized in Table 1. The expanded graphite sheets afterpurification treatment showed little changes in bulk density or in sheetthickness and thus were comparable in these respects to the sheetsbefore purification treatment.

The impurity content of each expanded graphite sheet after purificationtreatment was determined by weighing at least 15 g of the high-purityexpanded graphite sheet in a porcelain crucible, placing the crucible inan electric furnace, heating it at 850° C. for 48 hours, and calculatingthe impurity content from the mass before heating and the mass afterheating.

TABLE 1 Number Bulk Sheet of sheets Impurity density thickness inShaping Flexibility content (g/cm³) (mm/sheet) laminate method (times)(ppm) Example 1 1.3 0.5 1 Thomson die 12 4 Example 2 1.3 0.5 10 Thomsondie 12 4 Example 3 1.0 0.4 1 Thomson die 10 3 Example 4 1.0 0.4 10Thomson die 10 3 Example 5 0.7 0.4 1 Thomson die 10 2 Example 6 0.7 0.410 Thomson die 10 2 Example 7 1.3 0.5 1 Water jet 12 3 Example 8 1.3 0.510 Water jet 12 3 Example 9 1.0 0.4 1 Water jet 10 2 Example 10 1.0 0.410 Water jet 10 2 Example 11 0.7 0.4 1 Water jet 10 1 Example 12 0.7 0.410 Water jet 10 1 Example 13 1.3 0.2 1 Thomson die 20 3 Example 14 1.30.2 10 Thomson die 20 3 Example 15 1.0 0.2 1 Thomson die 18 2 Example 161.0 0.2 10 Thomson die 18 2 Example 17 0.7 0.2 1 Thomson die 10 1Example 18 0.7 0.2 10 Thomson die 10 1 Example 19 1.3 1.0 1 Water jet 105 Example 20 1.3 1.0 10 Water jet 10 5 Example 21 1.0 1.0 1 Water jet 104 Example 22 1.0 1.0 10 Water jet 10 4 Example 23 0.7 1.0 1 Water jet 102 Example 24 0.7 1.0 10 Water jet 10 2 Example 25 1.3 0.5 5 Water jet 121 Compar. Ex. 1 1.5 0.5 1 Thomson die 30 50 Compar. Ex. 2 1.5 0.5 10Thomson die 30 50 Compar. Ex. 3 1.3 0.1 1 Thomson die 1 1 Compar. Ex. 41.3 0.1 10 Thomson die 1 1 Compar. Ex. 5 1.3 1.2 1 Thomson die 5 30Compar. Ex. 6 1.3 1.2 10 Thomson die 5 30 Compar. Ex. 7 1.3 0.5 5 Cutter12 30 Compar. Ex. 8 0.6 0.4 10 Water jet 1 2 Compar. Ex. 9 0.3 0.4 10Water jet 1 1 Compar. Ex. 10 0.2 0.4 10 Water jet 1 1 Compar. Ex. 11 0.60.4 1 Water jet 1 2

As is evident from Table 1, those expanded graphite sheets having a bulkdensity of 0.7 to 1.3 g/cm³ and a thickness of 0.2 to 1.0 mm per sheetshowed only slight decreases in flexibility after purification treatmentand could be prevented from decreasing in purity.

A silicon single crystal growth experiment was carried out using thehigh-purity expanded graphite sheet obtained in Example 25 andComparative Example 3 each as an intervening liner in a CZ apparatus.When the liner produced in Example 25 was used, it was confirmed by theeye that the area of yellowed portions suggesting the formation ofsilicon carbide was smaller as compared with the case of the linerproduced in Comparative Example 3. This resulted from cracking of thehigh-purity expanded graphite sheet of Comparative Example 3 due to lackof flexibility.

When a plurality of expanded graphite sheets having a bulk density of0.7 to 1.3 g/cm³ and a thickness of 0.2 to 1.0 mm per sheet are formedinto a laminate and the late is worked into a desired shape, noimpairment in flexibility is found after treatment for attaining highpurity. In addition, when the laminate is shaped by water jet working,the working time can be reduced to 1/10 or shorter as compared with thatin the prior art and the impurity content can also be reduced to 1/10 orbelow. Therefore, the products are useful in the semiconductor-relatedindustries as those carbon crucible liners for use in CZ apparatus, CVDovens and the like which are required to have high purity andflexibility and, in the nuclear industry-related fields, as in-core orin-pile parts required to be flexile and highly pure.

1. A method of producing flexible, high-purity expanded graphite sheetswhich comprises forming an expanded graphite sheet having a bulk densityof 0.7 to 1.3 g/cm³ into a desired shape and then subjecting the shapedsheet to treatment for attaining high purity.
 2. A method of producingflexible, high-purity expanded graphite sheets which comprises forming aplurality of expanded graphite sheets each having a bulk density of 0.7to 1.3 g/cm³ into one and the same desired shape simultaneously in alaminate form, followed by treatment for attaining high purity.
 3. Themethod of producing flexible, high-purity expanded graphite sheets asdefined in claim 1, wherein the shaping of expanded graphite sheets iscarried out by at least one method selected from the group consisting ofworking on a slitting machine, punching using a Thomson die, water jetworking, and laser working.
 4. The method of producing flexible,high-purity expanded graphite sheets as defined in claim 2, wherein theshaping of expanded graphite sheets is carried out by at least onemethod selected from the group consisting of working on a slittingmachine, punching using a Thomson die, water jet working, and laserworking.
 5. The method of producing flexible, high-purity expandedgraphite sheets as defined in claim 1, wherein the expanded graphitesheet before purification treatment has a thickness of 0.2 to 1.0 mm. 6.The method of producing flexible, high-purity expanded graphite sheetsas defined in claim 2, wherein the expanded graphite sheet beforepurification treatment has a thickness of 0.2 to 1.0 mm.
 7. The methodof producing flexible, high-purity expanded graphite sheets as definedin claim 1, wherein the expanded graphite sheet after purificationtreatment has an impurity content not exceeding 10 ppm.
 8. The method ofproducing flexible, high-purity expanded graphite sheets as defined inclaim 2, wherein the expanded graphite sheet after purificationtreatment has an impurity content not exceeding 10 ppm.
 9. The method ofproducing flexible, high-purity expanded graphite sheets as defined inclaim 5, wherein the expanded graphite sheet after purificationtreatment has an impurity content not exceeding 10 ppm.
 10. The methodof producing flexible, high-purity expanded graphite sheets as definedin claim 6, wherein the expanded graphite sheet after purificationtreatment has an impurity content not exceeding 10 ppm.
 11. The methodof producing flexible, high-purity expanded graphite sheets as definedin claim 5, wherein the thickness is 0.3 to 0.9 mm.
 12. The method ofproducing flexible, high-purity expanded graphite sheets as defined inclaim 6, wherein the thickness is 0.3 to 0.9 mm.
 13. The method ofproducing flexible, high-purity expanded graphite sheets as defined inclaim 5, wherein the thickness is 0.5 to 0.8 mm.
 14. The method ofproducing flexible, high-purity expanded graphite sheets as defined inclaim 6, wherein the thickness is 0.5 to 0.8 mm.
 15. The method ofproducing flexible, high-purity expanded graphite sheets as defined inclaim 1, wherein the bulk density is 0.8 to 1.3 g/cm³.
 16. The method ofproducing flexible, high-purity expanded graphite sheets as defined inclaim 2, wherein the bulk density is 0.8 to 1.3 g/cm³.
 17. The method ofproducing flexible, high-purity expanded graphite sheets as defined inclaim 1, wherein the bulk density is 0.9 to 1.3 g/cm³.
 18. The method ofproducing flexible, high-purity expanded graphite sheets as defined inclaim 2, wherein the bulk density is 0.9 to 1.3 g/cm³.
 19. The method ofproducing flexible, high-purity expanded graphite sheets as defined inclaim 1, wherein said purification treatment comprises heating the sheetat 2,000° C. or above in a halogen atmosphere.
 20. The method ofproducing flexible, high-purity expanded graphite sheets as defined inclaim 2, wherein said purification treatment comprises heating the sheetat 2,000° C. or above in a halogen atmosphere.